The present invention relates to the field of non-invasive characterization of the nature of materials and, more particularly, to an apparatus and method for determining the moisture content of a bulk of non-homogeneous material.
Many different types of synthetic and organic materials are the basis for the formation of many different manufactured products. These materials must be gathered, transported and stored before being used in the manufacturing process. The manufacturing process itself may require multiple procedures, first to prepare the raw material, and then to use the processed material in the formation of the actual product. Many of these procedures are dependent upon the moisture content of the material. If the moisture content is too high, for example, the material may decompose during storage and transportation, before it can be used. If the moisture content is too low, processing and use of the material may be difficult.
Synthetic and organic non-homogenous materials whose behavior depends upon their moisture content include cotton, paper, wool, seeds, pharmaceuticals and synthetic fibers. For example, cotton is processed to separate the desired cotton from contaminating materials such as seeds, and is then spun into fibers for use in textile manufacture. For such processing and spinning to be successful, the cotton fibers should have an even moisture content that is neither too high nor too low. For example, fibers with low moisture content are weaker, breaking more frequently.
Cotton is typically transported and stored in the form of large bales or cases, hereinafter collectively referred to as modules. In addition, certain manufacturing processes entail processing of synthetic or natural materials, such as cotton, in a continues flow of bulk material, as on a conveyer belt system. The optimum moisture content of the cotton fibers for the production of textiles is from 6.5 to 8% before spinning and between 6-20% once packaged as a module, depending upon the requirements of the subsequent processing steps. Thus, effective moisture control in the textile mill depends upon accurate measurement of the moisture content of the fibers of the modules or the continuos bulk flow.
Non-invasive moisture measurements may be performed using microwave radiation. Typically, a microwave radiation source is located on one side of the cotton module, and a receiving antenna is located on the opposite side of the module. The radiation source beam is transmitted through a portion of the module and is received by the receiving antenna, which then produces a signal. This signal is used to determine the moisture content of that portion of the module and the mass uniformity of the module.
When the material being analyzed is of very high density and relatively uniform internal structure, such as the sheets of paper constituting the paper web output of a paper making machine, the moisture on the surface of the material acts as a mirror, particularly when the moisture content is relatively high (greater than 20%). In such a circumstance, incident microwave radiation may be reflected of off the surfaces of the material in a predictable manner, and the difference between reflected radiation and transmitted radiation (transmitted radiation being the radiation passing through the substance under analysis) can be used to measure moisture content. A device for measuring moisture content in this manner has been disclosed in U.S. Pat. No. 4,789,820 to Parrent Jr et al. Such a device, however, is unsuitable for use with poorly reflective materials, that is, materials of relatively non-uniform internal structure, lower moisture content, and lower density, such as modules of cotton, wool, seeds, pharmaceuticals and synthetic fibers.
Typically, microwave measurement of the moisture content of modules of non-uniform material is achieved by analyzing the characteristics of the attenuated transmitted microwave signal only, after it has passed through the module under analysis. A method for performing such moisture measurements is disclosed in U.S. Pat. No. 5,621,330 to Greenwald et al, which patent is incorporated herein. In Greenwald et al, the phase shift of the microwaves received by the receiving antenna is analyzed so as to determine the density of the material under analysis. The moisture content of the material is then calculated from the density data in conjunction with data describing the attenuation of the microwaves received by the receiving antenna, that is, the transmitted microwaves.
Calculating moisture based on measuring the phase shift of transmitted microwaves, however, suffers from several deficiencies:
1. The phase shift measuring apparatus is only capable of describing phase shifts within the phase region between 0 and 2.pi.. Depending on the nature of the material being analyzed, however, the true phase shift is often in a region greater than 2.pi.. As such, it is necessary to determine the phase region before calculating the phase shift for a transmitted microwave, so as to perform an appropriate correction. The determination of phase region is performed by analyzing the progressive change in phase shift which occurs as the leading edge of the module under analysis first enters the beam of microwave radiation being emitted by the moisture measuring device, inducing an "edge effect" on the received microwave signal. In the case of a continuos flow of bulk material, however (as opposed to individual modules), the absence of frequent leading edge effects precludes frequent calibrations to compensate for changes in phase region, thus rendering phase shift calculations inaccurate. As such, phase shift measuring devices are only suitable for use when individual modules are being analyzed, but cannot be used for analyzing a continues flow of bulk materials.
2. As each component of the phase shift measuring apparatus induces a phase shift of its own when processing the recorded signal, it is necessary to compensate for this effect when first constructing the device. As it is not feasible to recheck and adjust the phase region compensation once the device is in use, it is desirable that the components of the device not undergo spontaneous "drift" to a significant degree over time. As such, it is necessary to use components of the highest quality when constructing the device. Phase shift measuring devices are thus expensive and, due to the possibility of drift occurring, potentially unreliable in the long term.
3. The need to perform phase region compensation and phase shift calculation requires that specialized components be incorporated into the device. As a result, the device is complex to manufacture, thus adding to its expense.
4. Due to resolution difficulties, measurement of phase shift is unreliable in low density materials.
There is thus a widely recognized need for a device for non-invasively measuring the moisture content of a non-homogenous material, which is cheaper, less complex, and more reliable than current devices which measure microwave phase shift, which is capable of analyzing materials of low density as well as higher density, and which is capable of analyzing a continues flow of bulk materials in addition to individual modules of bulk materials.